1. Field of the Invention
The present invention relates to a hydrogen storage alloy electrode for a sealed nickel/metal hydride battery and a method of manufacturing the same. More specifically, it relates to the suppression of a decrease in the charging reserve of the hydrogen-storage alloy electrode of the sealed nickel/metal hydride battery.
2. Description of the Prior Art
Recently, the demand for an alkali storage battery for use in an electric vehicle and the like is increasing high. Especially, an alkali storage battery attracts attention as a power source for an electric vehicle and the like, because a nickel/metal hydride battery has a large energy capacity, using a hydrogen-storage alloy electrode as a negative electrode and a nickel hydroxide electrode as a positive electrode.
Such a prior-art nickel/metal hydride battery in which a negative electrode is an electrode made of a hydrogen-storage alloy, is constructed as a so-called "positive electrode-regulated battery" by designing the theoretical capacity of the positive electrode to be smaller than that of the negative electrode, as illustrated in FIG. 1. This construction is based on the intention of causing the negative electrode to absorb oxygen produced by the positive electrode at the end of charging, and simultaneously securing a charging reserve (a chargeable surplus capacity component) Pc, whereby the production of hydrogen on the negative electrode side is reduced to prevent the internal pressure of the battery from rising, and the negative electrode is endowed with a discharging reserve (a dischargeable surplus capacity component) Pd to enlarge the effective discharge area of the negative electrode for the discharging mode of the battery. In the prior art, therefore, the theoretical capacity of the negative electrode is usually designed to be at least about 1.6 times as large as that of the positive electrode.
Meanwhile, the hydrogen-storage alloy electrode is formed in such a way that the powder of the hydrogen-storage alloy is first turned into a paste by mixing it with water and a tackifier or a binder whose raw material is an organic polymer material, and that the paste is subsequently packed into a metallic collector member made of, for example, a porous foamed-nickel material. A highly hydrophilic organic polymer material which is also favorable for the formation of the paste, is most suitable as the organic polymer material which is employed for the tackifier or the binder mentioned above. By way of example, polyvinyl alcohol (PVA), methyl cellulose (MC) or carboxymethyl cellulose (CMC) which contains the hydroxyl group (OH group), which are highly hydrophilic, are usually employed as the tackifier or the binder.
From the results of various experiments, however, the inventors have discovered the occurrence of a problem that the aforementioned organic polymer material containing the OH group oxidizes and decomposes in the positive electrode during the charging of the battery, thereby decreasing the charging reserve, and in turn, incurring the production of hydrogen from the negative electrode at the end of the charging cycle and raising the internal pressure of the battery. Incidentally, an increase in the internal pressure of a sealed nickel/metal hydride battery causes the problem of shortening the charging cycles lifetime of the battery, resulting in operation of the safety valve thereof.